The present invention relates to fluoroionophores containing a fluorophore covalently bound to calix[4]arene via a bridging group and to processes for their preparation. The invention relates also to a) a sensor for determining sodium ions especially in aqueous solutions, which sensor comprises the fluoroionophores in an active layer; b) a method for the qualitative or quantitative determination of sodium ions, especially in aqueous solutions, using the optical sensor; and c) a composition comprising fluoroionophores and polymers.
The optical determination of ions has recently gained greater importance, the presence or concentration of ions being measured, for example, by means of a change in the absorption or fluorescence of a suitable dye. The sensors, also called optrodes, generally consist of a transparent support material and an active layer. The active layer normally comprises a transparent hydrophobic polymer and a lipophilic plasticiser for the purpose of obtaining adequate diffusion of the ions and adequate solubility of the active components. Active components are a specific ionophore as a complexing agent for ions, a counterion for maintaining electrical neutrality, and an indicator substance which, as a result of a chemical change or a physical change in the environment, emits a measurable optical signal. The disadvantages of many such optical sensors are that their response times are too long, they are pH-dependent, their long-term stability is too low for multiple usage and their sensitivities are too low.
The response times may be shortened by covalent linkage of ionophore and fluorophore to form so-called fluoroionophores. Such fluoroionophores are known from WO 89/00997 and U.S. Pat. No. 4,367,072.
In J. Mater.Chem. 4(1), (1994), pp. 145-151, Perez-Jimenez et al. describe two novel fluoroionophores that comprise 4 anthracene units covalently bound to calix[4]arene via an amide or ester bond. Disadvantageous fluorescence-quenching effects and lower sensitivity and selectivity as a result of steric hindrance may occur as a result of the close adjacency of the four anthracene units in the molecule.
It has now, surprisingly, been found that calix[4]arenes that comprise only a fluorophore covalently bound already exhibit a high degree of sensitivity and selectivity and are excellently suitable for detecting sodium. It has also, surprisingly, been found that it is possible to bind selectively only a fluorophore covalently to calix[4]arenes, the selectivity of the ionophore being retained. The spacing between the fluorophore and the ionophore can unexpectedly be varied within a wide range by means of the bridging group, without the sensitivity being adversely affected. The ion affinity, which is a necessary precondition for operation as a sensor, is virtually unaltered as a result of the modifications to the calixarene.
The invention accordingly relates firstly to fluoroionophores of formula (I) 
wherein
R06 is H or substituted or unsubstituted C1-C20alkyl,
R6 is H or substituted or unsubstituted C1-C30alkyl or C1-C30alkoxy,
R1 is a bridging group, and
F is a residue of a fluorophore.
R06 is especially linear or branched C1-C12alkyl, more especially linear or branched C1-C8alkyl.
Examples of alkyl are methyl, ethyl and the position isomers of propyl, butyl, pentyl, hexyl, heptyl, octyl, nonyl and decyl.
In a preferred embodiment, R06 is H or C1-C4alkyl.
More especially R06 is tertiary butyl or ethyl.
R6 is especially linear or branched C1-C10alkyl, more especially linear or branched C1-C4alkyl.
Examples of alkyl are methyl, ethyl and the position isomers of propyl, butyl, pentyl, hexyl, heptyl, octyl, nonyl and decyl.
In a preferred embodiment, R6 is H or C1-C4alkyl.
More especially R6 is tertiary butyl.
The bridging group R1 may contain in the chain from 1 to 30 atoms, preferably from 1 to 20 atoms and especially from 1 to 12 atoms, selected from the group C, O, S and N. The bridging group is preferably a hydrocarbon radical that may be interrupted by one or more hetero atoms from the group O, S and N. For adequate intramolecular interaction between fluorophore and ionophore in the same molecule it may be expedient to select a short bridging group, for example a bridging group having from 1 to 6, preferably from 1 to 4, atoms in the chain.
The bridging group R1 may correspond to formula (II)
xe2x80x94X1xe2x80x94(R3)rxe2x80x94X2xe2x80x94xe2x80x83xe2x80x83(II)
wherein X1 is xe2x80x94Oxe2x80x94 or xe2x80x94NR5,
X2 is a direct bond or is selected from the groups xe2x80x94Oxe2x80x94, xe2x80x94Sxe2x80x94, xe2x80x94NR5xe2x80x94, xe2x80x94C(O)xe2x80x94Oxe2x80x94, xe2x80x94Oxe2x80x94C(O)xe2x80x94,
xe2x80x94Oxe2x80x94C(O)xe2x80x94Oxe2x80x94, xe2x80x94SO2xe2x80x94Oxe2x80x94, xe2x80x94Oxe2x80x94SO2xe2x80x94, xe2x80x94Oxe2x80x94SO2xe2x80x94Oxe2x80x94, xe2x80x94NR5xe2x80x94C(O)xe2x80x94, xe2x80x94C(O)xe2x80x94NR5xe2x80x94, xe2x80x94NR5xe2x80x94C(O)xe2x80x94Oxe2x80x94,
xe2x80x94Oxe2x80x94C(O)xe2x80x94NR5xe2x80x94, xe2x80x94NR5xe2x80x94C(O)xe2x80x94NR5xe2x80x94, xe2x80x94NR5SO2xe2x80x94, xe2x80x94SO2xe2x80x94NR5xe2x80x94, xe2x80x94NR5xe2x80x94SO2xe2x80x94Oxe2x80x94, xe2x80x94Oxe2x80x94SO2NR5xe2x80x94 and
xe2x80x94NR5SO2xe2x80x94NR5xe2x80x94,
R5 is H or C1-C30alkyl, C5- or C6-cycloalkyl, C5- or C6-cycloalkylmethyl or -ethyl, phenyl,
benzyl or 1-phenyleth-2-yl,
R3 is a divalent bridging group,
r is 0 or 1, with the proviso that r is 1 when X2 is one of the mentioned groups.
When R5 is alkyl it has preferably from 1 to 6 carbon atoms and especially from 1 to 4 carbon atoms. Examples include methyl, ethyl, n-propyl, isopropyl, butyl, hexyl and octyl. When R5 is cycloalkyl it is preferably cyclohexyl, and when R5 is cycloalkylmethyl it is preferably cyclohexylmethyl. In a preferred embodiment, R5 is H or C1-C4alkyl.
The divalent bridging group R3 is preferably a hydrocarbon radical having preferably from 1 to 30 carbon atoms, more preferably from 1 to 18 carbon atoms, especially from 1 to 12 carbon atoms and more especially from 1 to 8 carbon atoms, and is unsubstituted or mono- or poly-substituted by C1-C4alkyl, C1-C4alkoxy or by xe2x95x90O. The hydrocarbon radical may also be interrupted one or more times by hetero atoms selected from the group xe2x80x94Oxe2x80x94, xe2x80x94Sxe2x80x94 and xe2x80x94NR5xe2x80x94 wherein R5 is preferably H or C1-C4alkyl.
The divalent bridging group R3 may be, for example, C1-C20alkylene, preferably C2-C12alkylene, which may be linear or branched. Examples include methylene, ethylene, 1,2- or 1,3-propylene, 1,2-, 1,3- or 1 ,4-butylene, pentylene, hexylene, octylene, dodecylene, tetradecylene, hexadecylene and octadecylene.
The divalent bridging group R3 may be, for example, polyoxaalkylene having from 2 to 12, especially from 2 to 6, and more especially from 2 to 4, oxaalkylene units and from 2 to 4, preferably 2 or 3, carbon atoms in the alkylene radical. R3 is especially polyoxaethylene or polyoxapropylene having from 2 to 6 oxaalkylene units.
The divalent bridging group R3 may be, for example, C5-C12-, especially C5-C8- and more especially C5- or C6-cycloalkyl, such as, for example cyclopentylene, cyclohexylene, cyclooctylene or cyclododecylene.
The divalent bridging group R3 may be, for example, C5-C12-, especially C5-C8- and more especially C5- or C6-cycloalkyl-C1-C12- or preferably xe2x80x94C1-C4-alkyl. Examples include cyclopentyl-CnH2nxe2x80x94 and cyclohexyl-CnH2nxe2x80x94, wherein n is from 1 to 4. -Cyclohexyl-CH2xe2x80x94 is especially preferred.
The divalent bridging group R3 may be, for example, C5-C12-, especially C5-C8- and more especially C5- or C6-cycloalkyl-(C1-C12alkyl)2- or preferably C1-C4alkyl)2. Examples include cyclopentyl-(CnH2nxe2x80x94)2 and cyclohexyl-(CnH2nxe2x80x94)2, wherein n is from 1 to 4. xe2x80x94CH2-Cyclohexyl-CH2xe2x80x94 is especially preferred.
The divalent bridging group R3 may be, for example, C6-C14arylene, especially C6-C10-arylene, for example naphthylene or especially phenylene.
The divalent bridging group R3 may be, for example, C7-C20aralkylene, especially C7-C12-aralkylene. Arylene-CnH2nxe2x80x94 wherein arylene is naphthylene or especially phenylene and n is from 1 to 4 is preferred. Examples are benzylene and phenylethylene.
The divalent bridging group R3 may be, for example, arylene-(CnH2nxe2x80x94)2xe2x80x94 wherein arylene is preferably naphthylene or especially phenylene and n is from 1 to 4. Examples include xylylene- and phenylene-(CH2CH2)2xe2x80x94.
In a preferred embodiment, the bridging group R1 is preferably xe2x80x94NR5xe2x80x94 wherein R5 is H, r is 0 and X2 is a direct bond.
The fluorophores from which F in formula (I) is derived are composed preferably of carbonyl groups, Cxe2x80x94C double bonds and aromatic rings, more especially condensed ring systems, such as naphthalenes, anthracenes, benzofurans, benzodiazines, benzotrioxazines, benzotriazepines, pyrenes and coumarins.
The fluorophores from which F in formula (I) is derived may be fluorescein or derivatives thereof, for example fluorescein derivatives of the formulae: 
or rhodamines or derivatives thereof, for example of the formulae: 
or acridines or derivatives thereof, for example of formula (V): 
wherein R01 and R02 are each independently of the other H or linear or branched C1-C30-alkyl and R03 is H or C1-C6alkyl.
The fluorophore is derived preferably from acridine or rhodamine or derivatives thereof, especially from 3,6-diaminoacridines.
The fluoroionophores to be used according to the invention are preferably compounds of formula (Ia) 
wherein R1, R06 and R6 have the meanings and preferred meanings indicated hereinbefore for the compound of formula (I), and R2 is xe2x80x94NR01R02 wherein R01 and R02 are as defined hereinbefore for formula (V), and the minimum carbon atom content is preferably at least 20 carbon atoms.
Especially preferred are fluoroionophores of formula (Ia) in which the bridging group R1 corresponds to formula (II)
xe2x80x94X1xe2x80x94(R3)rxe2x80x94X2xe2x80x94xe2x80x83xe2x80x83(II)
wherein X1 is as defined hereinbefore for the compound of formula (II), X1 preferably being NH, and X2 is a direct bond, r is 0 and R6 has the meanings and preferred meanings indicated hereinbefore for the compound of formula (I), and R2 has the meanings and preferred meanings indicated hereinbefore for formula (Ia).
Especially preferred are fluoroionophores of formula (Ia) in which R6 is preferably linear or branched C1-C10alkyl, especially linear or branched C1-C4alkyl, more especially tertiary butyl, and R1 has the meanings and preferred meanings indicated hereinbefore for the compound of formula (I) and R2 has the meanings and preferred meanings indicated hereinbefore for the compound of formula (Ia).
Especially preferred are fluoroionophores of formula (Ia) in which R06 is preferably linear or branched C1-C10alkyl, especially linear or branched C1-C4alkyl, more especially tertiary butyl or ethyl, and R1 has the meanings and preferred meanings indicated hereinbefore for the compound of formula (I) and R2 has the meanings and preferred meanings indicated hereinbefore for the compound of formula (Ia).
Especially preferred are fluoroionophores of formula (Ia) in which R2 is xe2x80x94NR01R02 wherein R01 and R02 are each independently of the other especially H or linear or branched C1-C10alkyl, more especially H or linear or branched C1-C7alkyl, and R1 and R6 have the meanings and preferred meanings indicated hereinbefore for the compound of formula (I).
More especially preferred are fluoroionophores of formula (Ia) in which the bridging group R1 corresponds to formula (II)
xe2x80x94X1xe2x80x94(R3)rxe2x80x94X2xe2x80x94xe2x80x83xe2x80x83(II)
wherein X1 is as defined hereinbefore for the compound of formula (II), X1 preferably being NH, X2 is a direct bond and r is 0, and
wherein R6 is preferably linear or branched C1-C10alkyl, especially linear or branched C1-C4alkyl, more especially tertiary butyl, and
wherein R2 is xe2x80x94NR01R02 wherein R01 and R02 are each independently of the other especially H or linear or branched C1-C10alkyl, more especially H or linear or branched C1-C7alkyl.
There may be mentioned specifically, for example, the following fluoroionophores of the formula 
wherein R15 is C1-C20alkyl, for example methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl or octyl, and
fluoroionophores of the formula 
wherein R15 is C1-C20alkyl, for example methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl or octyl.
The invention relates also to a process for the preparation of a compound of formula (I) which comprises reacting an ionophore of formula (Ib) 
with a fluorophore of formula (Ic)
Yxe2x80x2xe2x80x94(R3)rxe2x80x94X2xe2x80x94Fxe2x80x83xe2x80x83(Ic),
wherein R6 and R06 are as defined hereinbefore for formula (I) and R3 and X2 are as defined hereinbefore for formula (II),
and Y in formula (Ib) is Cl or Br and Yxe2x80x2 in formula (Ic) is xe2x80x94OH or xe2x80x94NHR03 wherein R03 is H or C1-C6alkyl.
In order to prepare a compound of formula (I), the functional groups that are not to be reacted may first be protected by protecting groups. The chain can then be extended at that functional group Yxe2x80x2. Known methods are, for example, etherification, esteritication, amidation, urea formation and urethane formation.
The compounds of formulae (Ic) and (Ib) are advantageously used in equimolar amounts.
Protecting groups and methods of derivatising functional groups are known from organic chemistry textbooks (E. Breitmaier, Gxc3xcnther Jung; Organische Chemie II (1983); Georg Thieme Verlag Stuttgart, New York p. 342, 409ff).
The functional groups may be protected, for example, by derivatisation. Functional groups of the xe2x80x94XH type (Xxe2x95x90O, S, NH) may be protected by acylation; acyl derivatives or carbonic acid derivatives may thus be prepared. Carboxy-protecting groups are known from peptide synthesis, for example methyl or ethyl, dimethylethylene or ptoluenesulfonic acid. Protecting groups for amino functions may be, for example, benzyloxycarbonyl, tert-butyloxycarbonyl, ptoluenesulfonyl, 2-nitrophenylsulfenyl, trifluoroacetyl or fluorenyl-methoxycarbonyl.
The linkage via functional groups may be carried out in accordance with generally known methods. It is, in principle, also possible to convert any functional groups that are present into different functional groups, for example to convert xe2x80x94CH2OH groups by oxidation into carboxylic acids, carboxylic acids into amides or halides, amine groups into isocyanate groups, and alcohols or amines into carbonates or urethanes. It is also possible for alcohols or amines to be reacted first of all with halocarboxylic acids (for example chloroacetic acid). Chain-extenders, for example epoxides, azirine, diols, diamines, dicarboxylic acids or esters and diisocyanates, may also be employed one or more times in succession, thus determining the length of the bridging group in a defined manner. Those linkage methods and procedures are known and are described in the specialist literature.
The reactions may be carried out with customary inert organic solvents at temperatures of from 0xc2x0 C. to 200xc2x0 C.
Suitable inert solvents are, for example, aprotic solvents, which may be used alone or in the form of mixtures of at least two solvents. Examples are: ethers (dibutyl ether, tetrahydro-furan, dioxane, ethylene glycol monomethyl or dimethyl ether, ethylene glycol monoethyl or diethyl ether, diethylene glycol diethyl ether, triethylene glycol dimethyl ether), halogenated hydrocarbons (methylene chloride, chloroform, 1,2-dichloroethane, 1,1,1-trichloroethane, 1,1,2,2-tetrachloroethane), carboxylic acid esters and lactones (ethyl acetate, methyl propionate, ethyl benzoate, 2-methoxyethyl acetate, xcex3-butyrolactone, xcex4-valerolactone, pivalo-lactone), carboxylic acid amides and lactams (N,N-dimethylformamide, N,N-diethylform-amide, N,N-dimethylacetamide, tetramethylurea, hexamethylphosphoric acid triamide, xcex3-butyrolactam, xcex5-caprolactam, N-methylpyrrolidone, N-acetylpyrrolidone, N-methylcapro-lactam), sulfoxides (dimethyl sultoxide), sulfones (dimethylsulfone, diethylsulfone, tri-methylenesultone, tetramethylenesulfone), tertiary amines (N-methylpiperidine, N-methyl-morpholine), aliphatic and aromatic hydrocarbons, for example petroleum ether, pentane, hexane, cyclohexane, methylcyclohexane, benzene or substituted benzenes (chloro-benzene, o-dichlorobenzene, 1,2,4-trichlorobenzene, nitrobenzene, toluene, xylene) and nitriles (acetonitrile, propionitrile, benzonitrile, phenylacetonitrile).
The compounds of formula (I) may be isolated in customary manner by precipitation, crystallisation, distillation or extraction and, where appropriate, may be purified by means of recrystallisation or chromatography.
The acids formed in the reaction are advantageously bound with hydrogen halide acceptors, for example with alkali metal carbonates or with tertiary amines, especially with sterically hindered tertiary amines.
Further details of the preparation of a compound of formula (I) will be found in the Examples.
The fluoroionophores according to the invention are excellently suitable as active components in optical ion sensors for the detection of sodium ions by means of a change in fluorescence.
The invention relates also to a composition comprising fluoroionophores according to the invention and polymers.
Various hydrophilic and hydrophobic polymers are suitable for the composition, xe2x80x9chydrophobicxe2x80x9d indicating that the water content in the polymers is a maximum of 15% by weight, preferably a maximum of 10% by weight, especially a maximum of 5% by weight, and more especially a maximum of 3% by weight, based on the polymer. Advantageously the polymers have a mean molecular weight of at least 5,000, especially at least 10,000 and more especially at least 20,000 daltons, for example from 20,000 to 200,000 daltons, especially from 50,000 to 4 million daltons. The polymers must be sufficiently soluble in organic solvents for them to be mixed with the other components and for them to be processed to form layers using customary coating methods. In addition, the polymers must be permeable to ions. The glass transition temperature is preferably from xe2x88x921300 to 300xc2x0 C. The dielectric constant of the polymers at 100 Hz and room temperature is especially from 2 to 50, more especially from 5 to 15. The optical transparency is especially in the range from 350 to 1200 nm, more especially from 400 to 900 nm.
Suitable polymers are known to the person skilled in the art and may be homo- or copolymers, block polymers, graft polymers or polymer blends. The polymer blends may be composed of, for example, polymer components having high and low glass transition temperatures. The glass transition temperature can be adjusted, for example, by the polarity, chain length and content of the structural units. The polymers may be selected, for example, from the group of polyvinyl compounds and polyacrylates, polyesters, polyamides, polyethers, polyimides, polyester amides, polyamide imides, polyurethanes, polyether urethanes, polyester urethanes, polyureas, polyurethane ureas and polysiloxanes, it being possible for the polymers to contain ionisable basic groups (for example amino groups) or acidic groups (for example carboxylic or sulfonic acid groups), which may help to provide improved ion transport.
Examples of monomers for polyolefin preparation include acrylic acid, methacrylic acid, maleic acid, maleic acid anhydride, acrylic and methacrylic acid C1-C30esters, acrylic and methacrylic acid C1-C30amides or acrylic and methacrylic acid amide, vinyl esters of C1-C20carboxylic acids, acrylonitrile, styrene, xcex1-methylstyrene, vinyl chloride, vinyl fluoride, vinylidene chloride, vinyl ethers of C1-C30alcohols and polyurethanes.
Polyesters, polyester amides and polyamides are composed especially of C2-C12-di-carboxylic acids and C2-C18-diols or -diamines. Polyimides are composed especially of C2-C18tetracarboxylic acids and C2-C18diamines. Polyethers are composed especially of aliphatic C2-C12diols (1,2- or xcex1,xcfx89-linkage) or linear adducts of such diols and C8-C30diglycidyl ethers. Polyurethanes and polyureas are composed especially of C2-C18-diols or -diamines and C2-C20-diisocyanates and/or -triisocyanates. Polysiloxanes are composed especially of di-C1-C4alkylsilyidichlorosilanes.
In a preferred embodiment, the polymers are polyurethanes of polyethers of C3-C6-alkanediols and aliphatic, cycloaliphatic, cycloaliphatic-aliphatic, aromatic-aliphatic or aromatic C2-C20diisocyanates, for example of polytetrahydroturan and bis(p-diiso-cyanatocyclohexyl)methane (Tecoflex).
In another preferred embodiment, the polymers are copolymers containing from 10 to 90 mol %, especially from 20 to 80 mol %, more especially from 30 to 70 mol %, of identical or different structural units of formula III 
and from 90 to 10 mol %, especially from 80 to 20 mol %, more especially from 70 to 30 mol %, based on the polymer, of identical or different structural units of formula IV 
wherein R7 and R8 are each independently of the other H or C1-C4alkyl, X is xe2x80x94Oxe2x80x94 or xe2x80x94NR14xe2x80x94,
R9 is C6-C20alkyl and R14 is H or C1-C20alkyl;
R10 and R11 are each independently of the other H, F, Cl or C1-C4alkyl, R12 and R13 are each independently of the other H, F, Cl, C1-C4alkyl, xe2x80x94COOH, xe2x80x94COOxe2x80x94C1-C5alkyl, xe2x80x94CONHC1-C5alkyl or xe2x80x94CON(R14)C1-C5alkyl, or R12 is H and R13 is xe2x80x94CN, phenyl, chlorophenyl, C1-C12alkoxy or C2-C18acyloxy.
R7 is especially H or methyl and R8 is especially H. X is especially xe2x80x94Oxe2x80x94. R9 is especially C6-C18alkyl. Examples of R9 are hexyl, heptyl, octyl, 2-ethylhexyl, nonyl, decyl, dodecyl, tetradecyl, hexadecyl and octadecyl.
R10 is especially H or methyl, R1 is especially H and R12 is especially H. R13 is especially xe2x80x94CN, phenyl, xe2x80x94COOxe2x80x94C1-C4alkyl, C1-C4alkoxy or C2-C6acyloxy. Examples of acyloxy include acetyloxy, propionyloxy, butyroyloxy, pentanoyloxy and hexanoyloxy.
The invention relates also to a material comprising (a) a support and (b) a hydrophilic polymer, ionophore and fluorophore, wherein the active layer comprises an effective amount of a fluoroionophore of formula (I).
The support is preferably transparent and may be formed, for example, from a plastics material, such as, for example, polycarbonate or acrylic glass, mineral materials or glass and may be of any shape, for example in the form of plates, cylinders, tubes, strips or fibres. The support material may be hard or flexible. Glasses are preferred.
The thickness of the layer on the support may be, for example, from 0.01 to 100 xcexcm, especially from 0.1 to 50 xcexcm, especially from 0.1 to 30 xcexcm and more especially from 0.1 to 10 xcexcm.
Such layers may be prepared in a manner known per se, for example by dissolving the composition and, if desired, a homo- and/or co-polymer in a solvent, then casting to form a film and subsequently removing the solvent. After removal of the solvent the film can be released from the substrate and a free-standing membrane is obtained.
Other processes that may be used for the production of the membrane are those known from surface-coating technology, for example spin-coating, spraying or knife application methods. Spin-casting processes are preferred.
Suitable solvents include water, alcohols, ethers, esters, acid amides and ketones. Readily volatile solvents, especially tetrahydrofuran, or solvent mixtures are especially suitable.
The membrane may be transparent or slightly opaque. It is preferably transparent. The layer is preferably hydrophilic.
The invention relates also to an optical sensor comprising a support coated on at least one side with a hydrophilic polymer, ionophore and fluorophore, wherein the active layer comprises an effective amount of a fluoroionophore of formula (I).
The optical range in which the material as sensor can be excited extends from the ultra-violet range to the infrared range. The immobilised fluorophoreionophores to be used in accordance with the invention have very suitable absorption and emission wavelength ranges that allow the use of known economically priced, low-energy light sources, for example halogen or xenon lamps or light-emitting diodes. The preferred excitation source is a light-emitting diode having a wavelength of 400 nm or above. The detectors used to detect the fluorescence may be, for example, photodiodes. Commercially obtainable optical fibres may be used in the excitation and detection. The sensor may therefore be changed after use on a patient.
The optical sensor is suitable especially for the quantitative determination of sodium ions, in an aqueous environment preferably using fluorescence spectrometry. The determinations may be effected within short periods of time with a high degree of accuracy even in the case of low concentrations (for example extending from the millimolar range to the nanomolar range).
A very important advantage of the immobilised fluoroionophores is that they offer the possibility of carrying out measurements that are substantially independent of pH value. There is a much freer choice of fluoroionophores since proton exchange at the fluorophore is not necessary for the detection of ions. In addition, direct measurement of the solution to be analysed is possible, which is of considerable commercial advantage. If desired, however, it is also possible in some cases for the measurements to be carried out in buffered analysis solutions when, for example, fluorophores are used that result in signal change as a result of proton exchange.
The analyses may be carried out, for example, directly in body fluids (blood, urine, serum), natural waters or waste water, it being possible for cations that may interfere to be selectively bonded or removed beforehand. The composition according to the invention is suitable especially for determining in aqueous media physiological amounts of sodium which may be, for example, in the range from 50 mmol to 200 mmol.
In addition to the preferred fluorescence spectroscopy, other methods of optical measurement may also be used, for example surface plasmon resonance spectroscopy, absorption spectroscopy, reflection spectroscopy, interferometry or surface-enhanced Raman or fluorescence spectroscopy.
The invention relates also to a method for the optical determination of sodium ions in aqueous test samples, in which method a sensor according to the invention is brought into contact with the said aqueous test sample and then the change in the fluorescence of the fluorophore in the polymer layer is measured.
The method according to the invention may be carried out, for example, by fixing the support with the active polymer layer in an optical cell in which the active layer comes into contact with the test sample. The optical cell has a window through which the active layer can be irradiated for the purpose of excitation and through which the emitted fluorescence radiation can be measured using a spectrofluorometer. The wavelengths may be adjusted to provide maximum absorption for the irradiation and maximum emission for the fluorescence measurement. The intensity is measured as a function of time. The measuring system may be so arranged that the measurement is carried out discontinuously or continuously by, for example, pumping the test solution through the measuring cell. To determine unknown concentrations of sodium ions, the system may first be calibrated using test samples of known concentration by plotting the concentrations against the intensity of the fluorescence.
The invention relates also to the use of the optical sensor for the determination of sodium ions by fluorescence spectroscopy.